1. Field of the Invention
This invention relates to a fuse used in an automobile or the like for protecting a load circuit against an excess current.
2. Related Art
Fuses made of a copper alloy or the like have heretofore been used for protecting an excess current-flowing circuit such as a motor load circuit in an automobile and also for protecting a circuit when a large burst current due to a rare short circuit is produced. Usually, such a fuse has been provided in the form of a terminal with a fuse in which the fuse is formed integrally with a terminal portion, or in the form of a terminal with a fuse in which a fuse element is bonded to a terminal portion.
FIG. 6 is an exploded, perspective view of one example of such a conventional terminal with a fuse, in which a fuse element is bonded to a terminal portion.
In this Figure, the terminal 100 with a fuse comprises the fuse element 101, the terminal portion 102 comprising a pair of contact portions 102A each having a fuse element connection portion 102B formed at an upper end thereof, and a housing 103. Opposite ends of the fuse element 101 are bonded respectively to the two fuse element connection portions 102B of the terminal portion 102, and the fuse element 101 and the terminal portion 102 thus connected together are housed in the housing 103 of a synthetic resin or the like.
A cover 104 made of a transparent resin is removably attached to an upper end of the housing 103 for preventing dust and the like from intruding into the housing and for enabling the fusion of the fuse to be viewed with the eyes from the exterior.
A pair of mating terminals (not shown) connected to a load circuit are fitted in and connected to the pair of contact portions 102A, respectively, so that current flows into one contact portion 102A, flows through the fuse element 101, and then flows out of the other contact portion 102A. At this time, if an excess current larger than an operating current flows as a result of the occurrence of some abnormality, the temperature of the fuse element 101 is raised by the generation of Joule heat proportional to the product of the square of a current density and a resistance value, and when this exceeds a predetermined temperature, the fuse element 101 is fused to break the circuit.
Three kinds of fuse elements heretofore used will now be described with reference to FIGS. 7 to 9, respectively.
FIG. 7(a) is a top plan view of a fuse element 51, and the fuse element 51 comprises a fusible body 52 part of which is a constricted portion 53, and connection ends 54 formed at opposite ends of this fusible body, respectively. The connection ends 54 are connected to the fuse element connection portions 102B of FIG. 6, respectively. Since the cross-sectional area of the constricted portion 53 is smaller than that of the remainder of the fusible body 52, a current density of the constricted portion 53 is higher than that of the remainder of the fusible body 52, and therefore the constricted portion 53 can be fused easily (see Japanese Patent Unexamined Publication No. 60-127630).
As shown in a fusion characteristics diagram of FIG. 7(b), as compared with ideal fusion characteristics 55, the time required for fusion of a fuse element 57 with no constricted portion is longer, whereas a fuse element 56 with the constricted portion is advantageously getting close to the ideal characteristics 55 at a region of high current. Therefore, if a target fusion region 58 is that of large current, the fusion is effectively carried out.
FIG. 8(a) is a perspective view of a fuse element of another construction. This fuse element 61 comprises a fusible body 62 having a chip 63 of low-melting metal embraced by part thereof, and connection ends 64 formed at opposite ends of this fusible body, respectively (see Japanese Utility Model Unexamined Publication No. 59-66844).
When the temperature of the fusible body 62 reaches a melting point of the chip 63, the embraced chip 63 melts to form, together with the fusible body 62 of metal, an eutectic alloy. A melting point of this alloy is lower than that of the original fusible body 62, and therefore this enables the fusible body to be fused in a short time.
As shown in a fusion characteristics diagram of FIG. 8(b), when an excess current is relatively small, the time required for fusion of a fuse element 67 with no chip is longer as compared with ideal fusion characteristics 65, whereas a fuse element 66 with the chip is advantageously getting close to the ideal characteristics 65 at a region of low current. Therefore, if a target fusion region 68 is that of low current, the fusion is effectively carried out.
FIG. 9(a) is a perspective view of a fuse element of a further construction. In this Figure, the fuse element 71 has a fusion portion 73 of a smaller cross-sectional area at a portion thereof, and two heat-radiating plates 72 with a larger radiating area are provided respectively at opposite ends of the fusion portion, and connection ends 74 are provided outwardly of the two radiating plates 72, respectively (see Japanese Utility Model Unexamined Publication No. 61-11258).
The fusion portion 73 has a small cross-sectional area, and therefore a current density of this portion is high, and hence the temperature of this portion can be easily raised as described above; however, the radiating plates 72 disposed adjacent thereto perform a radiating effect to alleviate the temperature rise, thus adjusting a time period before the fusion takes place.
As shown in a fusion characteristics diagram of FIG. 9(b), when an excess current is at a medium current region, the time required for fusion of a fuse element 77 with no radiating plate is shorter as compared with ideal fusion characteristics 75, whereas a fuse element 76 with the radiating plates has features that the time required for fusion is relatively long, and that it is getting close to the ideal characteristics 75 at a region of low current. Therefore, if a target fusion region 78 is that of medium current, a desired fusion time is achieved.
Although the conventional fuses are effective if the region of use is specified as described above, the following problems have been encountered when the region of use is wide:
The fuse element with the constricted portion shown in FIG. 7 is reduced in cross-sectional area so that it can be instantaneously fused by an excess current such as a burst current, as described above. As a result, it has a disadvantage that even if the excess current is at a medium current region, the fuse element can be fused in a relatively short time.
In this case, even if a medium current slightly exceeding a stationary current flows as an excess current even for a short period of time immediately after the operation is started as in a motor load circuit of an automobile, the fuse is fused, thus inviting a problem that the starting operation is quite inconvenient.
In the fuse element with the chip shown in FIG. 8, a time delay is encountered before the chip 63 of low-melting metal is fused, and therefore there has been encountered a problem that the fuse element can not be easily fused if an excess current is a large current such as a burst current.
In the fuse element with the radiating plates shown in FIG. 9, if an excess current is at a low current region, the radiating effect by the radiating plates becomes a reverse effect to prevent a temperature rise of the fuse portion 73, which results in a drawback that the fusion is not achieved within a desired time period.
In this case, if a low current, that is, a minimum operating current for fusing the fuse or a current close to it, is caused to flow as an excess current for a long period of time, the whole of the terminal with the fuse is maintained at high temperature for a relatively long period of time before the fuse is raised in temperature to be fused, and therefore there is encountered a problem that neighboring case and cover are melted.